LAUSR

OBJECTIVE To analyze the correlation between C2 incidence (C2I) and cervical and thoracolumbar sagittal parameters and determine the effectiveness of C2I for evaluating global spinopelvic sagittal alignment. METHODS Whole spine standing lateral radiographs of 226 patients (mean age, 47.8 ± 11.3 years; 63% women) were analyzed. Spinopelvic parameters and incidence angles of inflection point were evaluated. The correlation between each parameter was analyzed using Pearson correlation coefficient and linear regression. Using C2I quartiles, 3 groups were compared to distinguish different alignment patterns by analysis of variance. RESULTS C2I correlated significantly with C0-2 lordosis (C02L), C2-7 lordosis (C27L), C0-7 lordosis , C2 slope (C2S), T1 slope-C27L, C2-7 sagittal vertical axis, and chin brow vertical angle (CBVA) in cervical parameters (r = -0.378, r = 0.533, r = 0.251, r = 0.688, r = 0.681, r = 0.278, and r = 0.351, respectively; P < 0.01) and with T1 incidence (T1I), pelvic incidence-lumbar lordosis mismatch, L1 incidence (L1I), pelvic tilt (PT), and pelvic incidence (PI) (r = 0.480, r = 0.516, r = 0.518, r = 0.635, and r = 0.392, respectively; P < 0.01) in thoracolumbar alignment parameters. C2I was estimated by the 2 following equations: C2I = 1.0C2S + 1.0PT and C2I = 0.98PI - 0.99LL + 0.98 thoracic kyphosis - 1.0C27L (R2 = 0.97, P < 0.001, respectively), with an excellent coefficient of determination. PI, PT, L1I, T1I, and C2S were increased significantly between groups using C2I quartiles. In the high C2I group, C27L and C0-7 lordosis had less lordosis; however, C02L had more lordosis and C2-7 sagittal vertical axis and CBVA showed higher than low C2I group. CONCLUSIONS C2I was significantly correlated with both cervical and thoracolumbar sagittal parameters, and different sagittal alignment patterns were presented regarding the amount of C2I. As the geometric summation from the pelvis to C2 vertebra, C2I would be a beneficial clue to connect correlation chains of spinal sagittal alignment.